


Filling the Space

by GhostGarrison



Category: Supernatural
Genre: Alternate Universe - High School, Asexual Castiel, Asexual Character, Asexuality, Established Dean Winchester/Sam Winchester, M/M, Misunderstandings, Polyamory
Language: English
Status: Completed
Published: 2015-01-20
Updated: 2015-01-20
Packaged: 2018-03-08 10:00:43
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 2,218
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/3205124
Author URL: https://archiveofourown.org/users/GhostGarrison/pseuds/GhostGarrison
Summary: <blockquote class="userstuff">
              <p>When Sam's best friend Castiel walks in on the two brothers doing more than brotherly things, Sam runs off after him and Dean assumes the worst about everything.</p>
            </blockquote>





	Filling the Space

Castiel was the last person Dean expected to get in on their little shindig.

Well, maybe he’d be the first on his list if Dean had ever thought that he and Sam were to grow from a twosome to a threesome. But here they are, fitting together so perfectly that Dean has no idea how they went without their third all this time.

Castiel started out as Sam’s weird friend in middle school, quickly becoming friends when they moved in and bonding over nerdy shit and studying and everything Dean doesn’t really enjoy. But it made him happy that his brother was happy, even if it wasn’t Dean putting that smile on his face.

Their more-than-brotherly relationship was to stay a secret, nothing was ever supposed to be spoken aloud even if it was just between Sam and Dean. They never talked about how lonely their beds feel without the other when John is home for the night, or how good Sam feels tucked into his arms at night, or how Dean knows all the right places to make his little brother gasp quietly into his pillow. No, no one was supposed to know any of this.

Until Castiel came along.

Granted, it wasn’t until Sam’s sophomore year of high school when the damn teen found out about them, years after they’d become friends.

And also, as much as Dean hates to admit it, it’s all his own fucking fault that their secret is out.

It all started when Sam wanted a sleepover—yeah, a god damn sleepover, what is he, twelve?—and Castiel came over to their tiny house and stayed the night. The evening wasn’t really that bad. They ordered pizza and watched movies and Dean had to sit at an unbearable distance from his brother on the couch. He kept on looking over, paranoid that Castiel would make some move on Sam. Their hands were rather close together, and somehow Dean could have sworn that Castiel’s hand was sliding closer as the evening went on…

But no, it wasn’t until around midnight when they had all gone off to bed that Dean slipped into Sam’s room after checking that Cas was still on the couch, dozing peacefully under a spare blanket. Sam jolted awake when the mattress dipped beside him, but didn’t say a word except for rolling over and letting Dean in.

Dean whispered something in his little brother’s ear about “hated how I couldn’t touch you all night,” and something along the lines of “gonna fill my quota before the morning.” Sam hummed at his touch, letting his brother run his hands over his chest, arching into the welcomed contact. It was only when Dean had Sam on his arms and knees, with a hand is around his dick and was jacking Sam into oblivion when they heard the door click open.

"Sam, I—"

Dean’s head whipped around and Sam flinched under his touch, face still shoved into the pillow. Castiel stood in the doorway in his pajamas—flannel pants and one of _Sam’s_ shirts, Dean’s mind raged at the sight of it—his eyes wide and jaw slack. He stared for a moment before pulling the door closed, leaving the brothers in private again.

"Fuck," came a muffled curse through the pillow.

Fuck indeed, Dean agreed. They were being quiet. Castiel was asleep on the couch. He knew he was free to help himself to the kitchen, he knew where the bathroom was. What could he have wanted at nearly one in the morning by coming into Sam’s room?

Dean let off of his little brother and Sam’s face emerged from the folds of the pillow with a deep breath. He sat back on his heels, face hidden as his head hung low in shame.

"We have to talk to him," Sam mumbled quietly, unable to look Dean in the eye.

Having had agreed with his brother, they both get fully dressed again in their pajamas before venturing out to the den where Castiel was supposed to be sleeping. Something in the pit of Dean’s stomach told him that they should have gone after him quicker, that he won’t be in the den or even in the house when they walk out there.

But he’s wonderfully mistaken, as the guy is sitting sideways on the couch, legs crossed on the cushions as he stares out the window at the street lit only by the single streetlight on the corner. It’s peaceful and the light streaming in lights up Castiel’s profile and makes Dean’s heart flutter, and for a moment he completely forgets the situation. He’s never quite noticed how goddamn cute his brother’s friend was.

But then the gravity of it all weighed on him, causing his steps to slow at the entrance to the den as Sam strode right past him, going to the couch to kneel on the cushions before Castiel, sitting on his feet. Dean watched as his brother leaned in to capture the boy’s attention, a conversation between friends that Dean automatically felt that he was invading in on.

"Cas," Sam began and blue eyes lit by the streetlamp turned to look at him, cheeks ablaze with red.

"I’m sorr—"

"No," Sam interrupted sternly, "I’m sorry. _We’re_ sorry.”

Castiel casted a weary glance toward Dean, frowning and closing his eyes. The movement of Sam’s chest told him that he was letting out a heavy but silent sigh and his brother looks over at him. Dean took that as the cue to leave, scampering off to his room and laying in bed for an hour, wide awake and brain jumbled up from what could possibly be happening down the hall.

It’s nearing two-thirty in the morning when Sam returns to the bedroom, but not alone. He’s towing a silent and distraught looking Castiel, face red from what looks like some tears. Sam stood him in the middle of the room, in the neutral space between the door and the bed, and it’s then that Dean realized they’re holding hands. He didn’t know what to make of it.

"I really like Sam," Castiel confessed, eyes cast down to the floor of the messy room, glancing up a few times but not staying that way for long.

Dean looked between them, taking in the statement and the sight of their fingers laced together. He shoots a look at Sam, whose face is unreadable. Why is he dragging Cas in here to tell him this? Does Sam like him back? Does Sam want to break up with him so he can be with Cas, someone who’s normal and not his lusty and incestuous brother?

The pit in Dean’s stomach grew deeper, darker. Sam deserved that. Sam deserved something normal, something good. And if he decided that is Castiel, then he should be supportive of it.

"And?" Sam prompted his friend softly, nudging him gently while squeezing his hand.

Dean could see the boy’s adam apple bob as he swallowed, no doubt gathering the courage to say whatever was coming next. Probably asking Dean to have Sam instead.

"And…" Castiel began, eyes darting between the two brothers. "And I like you too, Dean."

"What," was all Dean could say.

It wasn’t even a question. What did that mean? Cas liked both of them? What’s the purpose of this? Why the fuck—?

"Dean," Sam said, sitting beside him on the bed and motioning for Castiel to join them. Dean awkwardly moved over to make space for all three of them, keeping his distance between both of them despite wanting to desperately reach out towards Sam, to grab him and hold him close and not let him go.

His little brother reached forwards to thread his fingers through Dean’s so now he’s holding all three of them together in an arc on the mattress. Sam calmly explained that, yes, Castiel had a crush on both of them at the same time, but ‘not like that,’ which prompted another long explanation that Castiel is asexual and isn’t interested in sex whatsoever. But the thing that resonates with Dean the most—the thing that echoed through his head even after the conversation ended and they all crashed in separate beds and couches for the evening—was that both Sam and he are interested in pursuing a relationship. Together. With Dean. All three.

Dean stayed up until the sun rose, and it was all utterly strange because Dean hasn’t pulled an all nighter in his life other than that one time he drank too much Redbull. This time it’s his thoughts that kept him awake, mulling over the events of the night as he stared up at the stucco ceiling of his room and pondered the possibilities.

He loved his brother with everything he had, but it was clear that Sam deeply cared for—or even loved, judging by his actions tonight—Castiel. He didn’t seem adverse to the idea of being in a relationship together, all three of them. Hell, Dean liked Castiel a lot—he was cute, nerdy, and made Sam very happy.

Dean left his room the next morning with steely resolve, treading to the kitchen after following the smell of pancakes in the kitchen. Sam stood at the stove with a spatula in hand, looking up at him like a deer in the headlights. Dean swooped in to give him a kiss, just a quick peck on the mouth like he does every morning before he gets up for work and Sam gets up for school.

Sam seemed a little more settled at that, knowing that Dean still loves him and that the events of the night didn’t ruin what they had. But it’s Dean’s next actions that clearly spell out his answer.

Crossing the kitchen, Dean came to stand beside Castiel, who sat frozen at the kitchen table and staring at him with nervous, wide eyes. Dean bent, pressing his lips to Castiel’s chapped ones, kissing him a little longer and deeper than he did Sam, hopefully getting the message across.

The look on both of their faces was priceless, both slackjawed and surprised, staring at him curiously like they didn’t understand what happened.

"I’ll be taking a shower if you guys need me," Dean said, before turning tail and making his way back through the house to the bathroom. It was cowardly, dropping that on them and basically running to hide. But he didn’t know how to explain himself otherwise.

Dean had enough room in his heart for both of them, and that was that.

Working Castiel into their already established relationship has been certainly interesting. The boy has now inserted himself in Dean’s fantasies, mirroring his brother who has been there for years. It’s amazing and Dean always comes harder and faster with both Sam and Castiel in them, but he knows it’ll never become reality, not like it did with him and Sam.

Dean still has time with Sam without Castiel around, when he’s at his own home doing his own thing, so it’s not like Dean is having to go without, but he’s definitely noticed that sex with Sam isn’t the same without Castiel. They’ve never done it as a threesome, but it still feels like something is missing when it’s just the two of them.

He finally figures it out when Castiel comes over one day unannounced, just after Dean had fucked Sammy hard against the headboard with that strange emptyish feeling still pooling in his stomach on top of the usual heat. The guy just lets himself into the house as always, as quiet as a mouse, and while he knew what Sam and Dean had obviously just done, he still crawls into the space between them as the doze the afternoon away.

It finally clicks then, when the empty place in Dean’s soul feels filled and it’s Castiel that filled that empty place, even if he was the one to make it in the first place.

In his mind, Dean is reluctant to call them a threesome because not all of them are ‘like that’ as Castiel so elegantly puts it, so he googles other terms, finally stumbling over the term ‘triad,’ which seems to fit since they’re all practically entangled with each other in one way or another.

If you asked Dean half a year ago, he would have said he could have never pictured himself like this, laying on his bed with a boy he loves so dearly in each arm, one of which is his own brother and the other is one he loves despite having no sexual relationship with. He’s a sexual being, much more than Sam, but at the very same degree that Castiel is _not_ sexual.

In all honesty, Dean has no idea how someone couldn’t like sex or be attracted to nobody. Sam seems to understand more than him, so at least one of them does. But just because Dean doesn’t understand what’s going on in Castiel’s head doesn’t mean he doesn’t accept him, doesn’t mean he doesn’t respect his boundaries.

He loves Sam, his brother and lover, and he loves Castiel just as much, the one who completes their ‘triad of awesome’ and fills the remaining space in Dean’s heart.

To Dean, there’s nothing more perfect than this.

**Author's Note:**

> written for a prompt on Tumblr, find me there @ GhostGarrison


End file.
